Preventive vaccines represent one of the most successful chapters of modern medicine, having led to the worldwide eradication of smallpox and to the control of polio, measles and many other devastating infectious diseases. More recently, vaccines have become available that prevent cancer, and strong efforts are ongoing to exploit “vaccines” in a therapeutic fashion, raising hope for both infection and malignancy. Historically, vaccination strategies have comprised a variety of approaches: Starting with the use of wild type infectious agents and the auto-(re)-inoculation of tumor cells, followed by live-attenuated agents and killed tumor tissues, clinical medicine has over time moved more and more to the use of (inert) proteins and/or other extracts (commonly referred to as “antigen”) derived from infectious agents or tumors, respectively. This gradual process represents the search for safer vaccine formulation, often accompanied, however, by a relative loss in efficacy. In recent years the advancement of biological engineering has made possible yet an additional approach that currently is widely considered among the most promising ones: infectious agents serving as a “ferry” (called “vector”) are equipped with an antigen from the pathogen or tumor of choice. Thereby, the immune response of the vaccine recipient recognizes the antigen of interest in the context of a strongly immune-enhancing (“immunogenic”) context conferred by the vector.
The “vector approach” has also made possible the directed introduction of foreign genes into living cells at the level of tissue culture but also in multicellular organisms including man, and vectors can therefore also be exploited for the expression of genes in cultured cells or in gene therapy.
A variety of vectors are currently in experimental use, both for vaccination and gene therapy, with the ultimate goal of optimizing efficacy and safety for clinical application (vaccinology and gene therapy) or for biotechnology (gene transfer in cell culture).
As a common observation, vectors tend to share general traits of the organism, e.g. virus, they are derived from. The exploitation of a novel family of viruses for vector design promises therefore a novel combination of traits that may confer this new type of vector with unprecedented capabilities and corresponding applications in biomedical application. Vector design needs, however, to take into account the safety profile of the organism used, and must come up with a strategy of how to eliminate the organism's pathogenic potential in a manner that does not interfere with desirable traits such as immunogenicity for administration as a vaccine.
Arenaviruses in general and lymphocytic choriomeningitis virus (LCMV) in particular have been known for more than seventy years to elicit extraordinarily strong and long-lasting humoral and cell-mediated immune responses. Of note, though, protective neutralizing antibody immunity against the viral envelope glycoprotein (GP) is minimal, meaning that infection results in minimal antibody-mediated protection against re-infection if any. Also it has been firmly established for decades that owing to their non-cytolytic (not cell-destroying) nature, arenaviruses can, under certain conditions, maintain long-term antigen expression in animals without eliciting disease. Recently, reverse genetic systems for the manipulation of the infectious arenavirus genome (L. Flatz, A. Bergthaler, J. C. de la Torre, and D. D. Pinschewer, Proc Natl Acad Sci USA 103:4663-4668, 2006; A. B. Sanchez and J. C. de la Torre, Virology 350:370, 2006) have been described, but arenaviruses have not so far been exploited as vaccine vectors. Two major obstacles are mainly responsible: i) Arenaviruses can cause overwhelming infection which then can result in serious disease and immunosuppression. ii) The incorporation of foreign antigens of choice has not been possible.